Impeller

ABSTRACT

An impeller comprises a hub and blades extending therefrom. The blades include splitter blades interspersed between full blades. First flow channels are defined between pressure sides of the splitter blades and suction sides of the full blades. Second flow channels are defined between suction sides of the splitter blades and pressure sides of the full blades. A width of the first flow channels at leading edges of the splitter blades and at the hub is less than a width of the second flow channels at the leading edges of the splitter blades and at the hub. The width of the first flow channels at the leading edges of the splitter blades and at the tips is more than the width of the second flow channels at the leading edges of the splitter blades and at the tips.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, moreparticularly, to impellers used in such engines.

BACKGROUND OF THE ART

Impellers may be used as radial compressors in gas turbine engines. Animpeller has a hub and blades disposed therearound. When the impellerrotates about its rotational axis, a secondary flow that includesthree-dimensional vortical flow structures develops in blade passagesdue to the rotation of the flow and to the non-uniform inlet pressureprofiles. While main flow is responsible for extracting or providingenergy to the working fluid, the secondary flow, which is transverse tothe main flow, acts to reduce energy available for transfer to theworking fluid. The secondary flow creates flows that go from pressuresides to suction sides of the blades. The secondary flow thencontributes to tip leakage flow and to pre-mature flow blockage in theimpeller.

SUMMARY

In one aspect, there is provided an impeller comprising a hub, bladesextending from the hub to tips, the blades having pressure sides andsuction sides, the blades including splitter blades interspersed betweenfull blades, a chord length of the splitter blades less than a chordlength of the full blades, first flow channels defined between pressuresides of the splitter blades and suction sides of the full blades,second flow channels defined between suction sides of the splitterblades and pressure sides of the full blades, respective widths of thefirst and second flow channels at a given rotor location defined betweenadjacent splitter and full blades at the given rotor location, a widthof the first flow channels at leading edges of the splitter blades andat the hub less than a width of the second flow channels at the leadingedges of the splitter blades and at the hub, and the width of the firstflow channels at the leading edges of the splitter blades and at thetips more than the width of the second flow channels at the leadingedges of the splitter blades and at the tips.

In another aspect, there is provided an impeller comprising a hub,blades disposed on the hub, the blades having pressure sides and suctionsides extending on opposite sides of the blades from the hub toward tipsof the blades and from leading edges toward trailing edges spaced apartfrom the leading edges by chord lengths, the blades including fullblades and a splitter blade between adjacent full blades, a chord lengthof the splitter blade less than a chord length of the full blades, theadjacent full blades including pairs of a first full blade on a pressureside of the splitter blade and a second full blade on a suction side ofthe splitter blade, a leading edge and a trailing edge of the splitterblade extending from the hub toward a tip of the splitter blade, a ratioof a circumferential distance between the leading edge of the splitterblade and the second full blade over a circumferential distance betweenthe adjacent full blades at the leading edge of the splitter bladedecreasing from a value greater than 0.5 at the hub to a value less than0.5 at the tip of the splitter blade.

In yet another aspect, there is provided a method for operating animpeller, the method comprising: dividing a flow of working fluidreceived by the impeller within flow channels circumferentiallydistributed around the impeller; for each of the flow channels: dividingthe flow of working fluid in the flow channels into a first flow channelportion and in a second flow channel portion; and at an upstreamlocation of the first and second flow channel portions, diverging agreater portion of the flow in the second flow channel portion relativeto the first flow channel portion by exposing the flow to a greaterflowing area in a radially-inward region of the second flow channelportion than in a radially-inward region of the first flow channelportion.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic tridimensional view of an impeller of the engineof FIG. 1;

FIG. 3 is a schematic enlarged view of the impeller of FIG. 2;

FIG. 4 is a schematic cross-sectional view following line 4-4 of FIG. 3illustrating leading edges of blades of the impeller of FIG. 2;

FIG. 5 is a schematic cross-sectional view following line 5-5 of FIG. 3illustrating trailing edges of the blades of the impeller of FIG. 2;

FIG. 6 is graph illustrating a variation of a circumferential distancebetween a full blade and an adjacent splitter blade of the impeller ofFIG. 2 along a hub of the impeller and along a tip of the splitter bladein function of a chord-wise position between a leading edge and atrailing edge of the splitter blade; and

FIG. 7 is a graph illustrating a variation of a circumferential distancebetween a full blade and an adjacent splitter blade of the impeller ofFIG. 2 along the leading and trailing edges of the splitter blade infunction of a span-wise position between the hub and the tip of thesplitter blade.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication a fan 12 through which ambient air is propelled, acompressor section 14 for pressurizing the air, a combustor 16 in whichthe compressed air is mixed with fuel and ignited for generating anannular stream of hot combustion gases, and a turbine section 18 forextracting energy from the combustion gases. In the embodiment shown,the compressor section 14 includes an impeller 20 also referred to as aradial compressor. The fan 12, the compressor section 14, and theturbine section 18 are configured for rotation about a longitudinal axis11 of the gas turbine engine 10.

Referring now to FIGS. 2-3, the impeller 20 is configured for rotationabout a rotational axis R, which, in the embodiment shown, is coincidentwith the engine longitudinal axis 11. The impeller 20 has a hub 22having an axial-to-radial shape. Stated otherwise, in use, a flow ofworking fluid circulates substantially tangentially to a surface 24 ofthe hub 22. The flow, in an upstream location 26 of the impeller 20, isaligned substantially axially relative to the axis R and, in adownstream location 28, is aligned substantially radially relative tothe axis R. In other words, the flow circulates generally parallel tothe axis R when entering the impeller 20 and generally perpendicularlyto the axis R, and away therefrom, when exiting the impeller 20. If theimpeller 20 is used as a turbine impeller instead of a compressorimpeller, this arrangement is opposite.

The impeller 20 further includes blades 30 disposed on the hub 22. Theblades 30 have pressure sides 32 and suction sides 34. The pressure andsuction sides 32 and 34 extend on opposite sides of the blades from thehub 22 toward tips 36 of the blades 30 and from leading edges 38 towardtrailing edges 40 of the blades 30. The blade leading and trailing edges38 and 40 are spaced apart from one another by chord lengths. The hub 22and the tips of the blades 36 are spaced apart from one another byspans.

In the embodiment shown, the blades 30 include full blades 42 andsplitter blades 44 circumferentially disposed in alternation around therotational axis R. Stated otherwise, in an embodiment, one of thesplitter blades 44 may be disposed between each set of adjacent two ofthe full blades 42. Chord lengths of the splitter blades 44 are lessthan chord lengths of the full blades 42. In the illustrated embodiment,the splitter blades chord lengths range between 50% to 80% of the fullblades chord lengths. An adjacent pair of the full blades 42 isillustrated and referred to as blades 42 a and 42 b herein, tofacilitate the understanding of the subsequent paragraphs. However, theconcepts described for blades 42 a and 42 b may apply to the other pairsof the adjacent blades 42.

The impeller 20 has a flow channel 46 between each adjacent two of thefull blades 42, which include the first full blade 42 a and the secondfull blade 42 b. The first full blade 42 a is disposed before the secondfull blade 42 b relative to a direction of rotation of the impeller 20around the axis R. The direction of rotation is illustrated by arrow A.The flow channels 46 are configured for receiving an incoming flow atthe upstream location 26 and for outletting the flow at the downstreamlocation 28. Along the flow channels 46, the flow changes direction fromgenerally axially, and parallel to the rotational axis R, to generallyradially, and perpendicular to the rotational axis R. A width of each ofthe flow channels 46 varies from the upstream 26 to the downstream 28locations. For any given position along the rotational axis R betweenthe upstream 26 and downstream 28 locations, the height of each of theflow channels 46 may be constant in a radial direction from the hub 22toward the blade tips 36. The width varies along the axis R. The widthof each of the flow channels 46 may increase from the upstream 26 to thedownstream 28 locations.

The blade leading edges 38 are circumferentially offset from the bladetrailing edges 40. Consequently, the flow circulating within the flowchannels 46 moves around the rotational axis R from the upstreamlocation 26 to the downstream location 28. In the case of a compressorimpeller, this rotation allows the impeller to transfer energy to theworking fluid. Alternatively, in the case of a turbine impeller, thisrotation allows the impeller to extract energy from the working fluid.

In the embodiment shown, each of the flow channels 46 is divided in twodownstream of the upstream location 26 by the splitter blades 44. Hence,the flow channels 46 each diverge into a first flow channel 46 a and asecond flow channel 46 b. The first flow channel 46 a is bounded by asuction side 34 of the first full blade 42 a, a pressure side 32 of oneof the splitter blades 44 adjacent the first full blade 42 a and by thehub 22. The second flow channel 46 b is bounded by a suction side 34 ofthe one of the splitter blades 44, by a pressure side 32 of the secondfull blade 42 b adjacent to the one of the splitter blades 44, and bythe hub 22.

In some circumstances, secondary flow, generally referred to asthree-dimensional vortical flow structures, develops in the first andsecond flow channels 46 a and 46 b due to flow turning and non-uniforminlet pressure profiles. While main flow is responsible for energyextraction/transfer, the secondary flow is transverse to the main flowand acts to reduce energy available for energy extraction/transfer. Thesecondary flow originates in a boundary layer that flows along theblades 30 and contains span-wise velocity gradient. When boundary flowis turned, traverse velocity components are introduced.

In some cases, the secondary flow creates cross flows that go from theblade pressure sides 32 to the blade suction sides 34. These flows arecarried off the adjacent suction side 34 from the hub 22 to the tips 36due to pressure difference and centrifugal force. The secondary flowinteracts with main blade tip clearance flow. This interaction mayresult in mixing loss and flow blockage. The combination of leakage andsecondary flow is observed to reach the entire passage and may createlarge flow blockage. Large flow blockage may lead to premature impellerinducer stalling, or affect performance of downstream components.

In the illustrated embodiment, the splitter blades 44 are rotated suchthat portions adjacent to the hub 22 are moved toward the suction sides34 of the adjacent full blades 42. At the hub 22, these offsets createlarger pitches between pressure sides 32 of the full blades 42 and theadjacent splitter blades 44 and smaller pitches between the adjacentsplitter blades 44 and the suction sides 34 of the full blades 42. Theincrease in pitch may tend to drive more of the secondary flow towardthe splitter suction sides while reducing flow toward the adjacent fullblade suction sides. With less flow toward the adjacent full bladesuction sides 34 there may be less flow centrifuged along the suctionsides, and hence there may be less flow to interact with tip leakageflow, and thus, there may be less mixing loss. At the splitter bladetips 36, the splitter blades 44 are rotated in the opposite directioncompared to their rotation at the hub 22. This increases the pitchesbetween the splitter blades 44 and the adjacent full blade suction sides34.

Referring now to FIGS. 2-5, standard splitter blades H are not part ofthe impeller 20 and are shown for illustration purposes only. Animpeller having the standard splitter blades H constitutes a baselineconfiguration against which performances of the disclosed impeller 20are compared. Each of the original splitter blades H is centered betweentwo adjacent ones of the full blades 42. In other words, acircumferential distance between one of the original splitter blades Hand the first full blade 42 a corresponds to a circumferential distancebetween the one of the original splitter blades H and the second fullblade 42 b at any axial location along the rotational axis R. It isunderstood that a circumferential distance between two blades is definedas a circumferential distance between a first location on one of the twoblades and a second location on the other one of the two blades, thesecond location circumferentially corresponding to the first location.

Referring more particularly to FIGS. 4-5, the standard splitter blades Hare shown in dashed lines whereas the splitter blades 44 are shown infull lines. At leading edges 56 of the splitter blades 44 (FIG. 4) awidth W1 of the first channels 46 a increases from the hub 22 towardtips 60 of the splitter blades 44. Consequently, still at the leadingedges 56, a width W2 of the second channels 46 b decreases from the hub22 toward the tips 60. An angle B′ between the splitter blades 44 andthe hub 22, at the splitter blade leading edges 56, is obtuse whereas anangle B1 between the original splitter blades H and the hub 22 is acuteat the splitter blade leading edges 56. An angle B″ between the splitterblades 44 and the hub 22, at the splitter blade trailing edges 58, isobtuse and has a value more than a value of an angle B2 between theoriginal splitter blades H and the hub 22 at the splitter blade trailingedges 58. In the illustrated embodiment, a difference between the anglesB′ and B1 ranges from 15 to 25 degrees and a difference between theangles B″ and B2 ranges from 5 to 15 degrees. In the embodiment shown,at the trailing edges 58 (FIG. 5), a width W3 of the first channels 46 aincreases from the hub 22 toward the splitter blade tips 60. A width W4of the second channels 46 b decreases from the hub 22 toward the tips60. Stated otherwise, a circumferential distance between the splitterblade leading edges 56 and the adjacent second full blades 42 bdecreases from the hub 22 toward the splitter blade tips 60. Acircumferential distance between the splitter blade trailing edges 58and the adjacent second full blade 42 b decreases from the hub 22 towardthe splitter blade tips 60.

Referring more particularly to FIG. 4, at the splitter blade leadingedges 56, the width W1 of the first flow channels 46 a increases fromthe hub 22 to the splitter blade tips 60 from less than half a foremostcombined width WF of two adjacent ones of the first and second flowchannels 46 a and 46 b to more than half of the foremost combined widthWF. Consequently, the width W2 of the second flow channels 46 bdecreases from the hub 22 to the splitter blade tips 60 from more thanhalf the foremost combined width WF to less than half of the foremostcombined width WF. In other words, a foremost ratio of thecircumferential distance between the leading edge 56 of one of thesplitter blades 44 and the second blade 42 b over a circumferentialdistance between the first and second full blades 42 a and 42 b at thesplitter blade leading edges 56 decreases from a value more than 0.5 atthe hub 22 to a value less than 0.5 at the tips 60. In a particularembodiment, the foremost ratio varies from 0.7 at the hub 22 to 0.3 atthe tips 60. In a particular embodiment, the foremost ratio varies at aconstant rate from the hub 22 to the tips 60.

Still referring to FIG. 4, in other words, at the hub 22, the width W1of the first flow channels 46 a at leading edges 56 of the splitterblades 44 is less than the width W2 of the second flow channels 46 b atthe leading edges 56 of the splitter blades 44. At the tips 60, thewidth W1 of the first flow channels 46 a at the splitter leading edges56 is more than the width W2 of the second flow channels 46 b at thesplitter leading edges 56.

Referring more particularly to FIG. 5, at the splitter blade trailingedges 58, the width W3 of the first flow channels 46 a increases fromthe hub 22 to the splitter blade tips 60 from half an aft most combinedwidth WA of the two adjacent ones of the first and second flow channels46 a and 46 b to more than half of the aft most combined width WA. Thewidth W4 of the second flow channels 46 b decreases from the hub 22 tothe tips 60 from half the aft most combined width WA to less than halfof the aft most combined width WA. In other words, an aft most ratio ofthe circumferential distance between the trailing edge 58 of the one ofthe splitter blades 44 and the adjacent one of the second full blades 42b over a circumferential distance between the first and second fullblades 42 a and 42 b at the trailing edges 58 of the splitter blades 44decreases from a value of 0.5 at the hub 22 to a value less than 0.5 atthe tips 60. In the embodiment shown, at the splitter blade tips 60, theforemost ratio is less than the aft most ratio and the angle B′ is morethan the angle B″.

Still referring to FIG. 5, in other words, at the hub 22, the width W3of the first flow channels 46 a at trailing edges 58 of the splitterblades 44 corresponds to the width W4 of the second flow channels 46 bat the trailing edges 58 of the splitter blades 44. At the tips 60, thewidth W3 of the first flow channels 46 a at the splitter trailing edges58 is more than the width W4 of the second flow channels 46 b at thesplitter trailing edges 58.

Referring now also to FIGS. 6 and 7, graphs illustrate a possiblevariation of a relative position of the splitter blades 44 relative tothe first and second full blades 42 a and 42 b. A relative positioncorresponds to a ratio of a circumferential distance between a givenlocation on one of the second blades 42 b and a circumferentiallycorresponding location on an adjacent one of the splitter blades 44 overa circumferential distance between the given location and acircumferentially corresponding location on an adjacent one of the firstblades 42 a. FIG. 6 illustrates a variation of the ratio between thesplitter blade leading and trailing edges 56 and 58 on the hub 22 and onthe splitter blade tips 60. FIG. 7 illustrates a variation of the ratiobetween the hub 22 and the splitter blade tips 60 on the splitter bladeleading and trailing edges 56 and 58.

As illustrated in FIG. 6, at the hub 22, the ratio decreases from avalue more than 0.5 at the splitter blade leading edges 56 to a value of0.5 between the leading and trailing edges 56 and 58. In the illustratedembodiment, the ratio reaches a value of 0.5 at about 60% of thesplitter blade chord lengths from the splitter blade leading edges 56.In the embodiment shown, at the hub 22, the ratio follows a monolithicdistribution. In the illustrated embodiment, at the splitter blade tips60, the ratio increases from about 0.7 at the leading edges 56 to about0.35 at the trailing edges 58. In the illustrated embodiment, the ratiomostly following a linear relationship at the tips 60.

Referring to FIGS. 3 and 6, at the hub, the width of the first flowchannels 46 a downstream of the splitter leading edges 56 is less thanthe width of the second flow channels 46 b along at least a portion ofthe chord length of the splitter blades 44. In the embodiment shown,this portion corresponds to about 60% of the splitter blade chordlengths from the splitter blade leading edges 56. At the splitter tips60, the width of the first flow channels 46 a downstream of the splitterleading edges 56 is more than the width of the second flow channels 46 balong at least a portion of the splitter blade chord lengths. In theembodiment shown, the portion corresponds to the entire length of thesplitter blades 44.

As illustrated in FIG. 7, the ratio for both the splitter blade leadingand trailing edges 56 and 58 decreases from 0.7 and 0.5 to 0.3 and 0.35,respectively, from the hub 22 toward the splitter blade tips 60. In theembodiment shown, the variation of the ratio is linear at the trailingedges 58. For the leading edges 56, the ratio decreases at a constantrate until it reaches 0.5 and increases thereafter such that the ratiochange more rapidly near the blade tips 60 . Other configurations arecontemplated.

Referring to all figures, to operate the impeller 20 a flow entering theimpeller 20 is first received within flow channels 46 circumferentiallydistributed around the impeller 20. Then, for each of the flow channels46 the flow of working fluid is divided in a first flow channel portion46 a and in a second flow channel portion 46 b. And, at the upstreamlocation 26, a greater portion of the flow is diverged in the secondflow channel portion 46 b relative to the first flow channel portion 46a by exposing the flow to a greater flowing area in a radially-inwardregion of the second flow channel portion than in a radially-inwardregion of the first flow channel portion. The flowing area of a givenregion of a flow channel corresponds to an area of a cross-section ofthe given region of the flow channel. The cross-section is taken along aplane that is perpendicular to the hub 22.

In the embodiment shown, at the upstream location 26, the flow isexposed to a greater flowing area in a radially-outward region of thefirst flow channel portion 46 a than in a radially-outward region of thesecond flow channel portion 46 b. At the downstream location 28, theflow is exposed to a greater flowing area in the radially-outward regionof the first flow channel portion 46 a than in the radially-outwardregion of the second flow channel portion 46 b. At the downstreamlocation 28, the flow is exposed to a flowing area that is greater inthe first flow channel portion 46 a than in the second flow channelportion 46 b.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. An impeller comprising a hub, blades extending from the hub to tips,the blades having pressure sides and suction sides, the blades includingsplitter blades interspersed between full blades, a chord length of thesplitter blades less than a chord length of the full blades, first flowchannels defined between pressure sides of the splitter blades andsuction sides of the full blades, second flow channels defined betweensuction sides of the splitter blades and pressure sides of the fullblades, respective widths of the first and second flow channels at agiven rotor location defined between adjacent splitter and full bladesat the given rotor location, a width of the first flow channels atleading edges of the splitter blades and at the hub less than a width ofthe second flow channels at the leading edges of the splitter blades andat the hub, and the width of the first flow channels at the leadingedges of the splitter blades and at the tips more than the width of thesecond flow channels at the leading edges of the splitter blades and atthe tips.
 2. The impeller of claim 1, wherein the width of the firstflow channels at trailing edges of the splitter blades and at the hubcorresponds to the width of the second flow channels at the trailingedges of the splitter blades and at the hub, the width of the first flowchannels at the trailing edges of the splitter blades and at the tipsmore than the width of the second flow channels at the trailing edges ofthe splitter blades and at the tips.
 3. The impeller of claim 1, whereinthe width of the first flow channels at the hub corresponds to the widthof the second flow channels at the hub at a given position between theleading edges and trailing edges of the splitter blades.
 4. The impellerof claim 3, wherein the given position corresponds to 60% of the chordlength of the splitter blades from the leading edges thereof.
 5. Theimpeller of claim 1, wherein the width of the first flow channels at thehub and downstream of the leading edges of the splitter blades is lessthan the width of the second flow channels at the hub along at least aportion of the chord length of the splitter blades.
 6. The impeller ofclaim 1, wherein the width of the first flow channels at the leadingedges of the splitter blades varies from 30% to 70% of a combined widthof the first and second flow channels at the leading edges.
 7. Theimpeller of claim 1, wherein angles between the hub and the splitterblades at the leading edges thereof are greater than angles between thehub and the splitter blades at trailing edges thereof.
 8. The impellerof claim 1, wherein the width of the first flow channels at the tips anddownstream of the leading edges of the splitter blades is more than thewidth of the second flow channels at the tips along at least a portionof the chord length of the splitter blades.
 9. An impeller comprising ahub, blades disposed on the hub, the blades having pressure sides andsuction sides extending on opposite sides of the blades from the hubtoward tips of the blades and from leading edges toward trailing edgesspaced apart from the leading edges by chord lengths, the bladesincluding full blades and a splitter blade between adjacent full blades,a chord length of the splitter blade less than a chord length of thefull blades, the adjacent full blades including pairs of a first fullblade on a pressure side of the splitter blade and a second full bladeon a suction side of the splitter blade, a leading edge and a trailingedge of the splitter blade extending from the hub toward a tip of thesplitter blade, a ratio of a circumferential distance between theleading edge of the splitter blade and the second full blade over acircumferential distance between the adjacent full blades at the leadingedge of the splitter blade decreasing from a value greater than 0.5 atthe hub to a value less than 0.5 at the tip of the splitter blade. 10.The impeller of claim 9, wherein a ratio of a circumferential distancebetween the trailing edge of the splitter blade and the second fullblade over a circumferential distance between the first and second fullblades at the trailing edge of the splitter blade decreases from a valueof 0.5 at the hub to a value less than 0.5 at the tip of the splitterblade.
 11. The impeller of claim 9, wherein a circumferential distancebetween the splitter blade and the first full blade at the hub and at agiven position between the leading edge and the trailing edge of thesplitter blade is equal to a circumferential distance between thesplitter blade and the second full blade at the hub and at the givenposition.
 12. The impeller of claim 11, wherein the given positioncorresponds to 60% of the chord length of the splitter blade from theleading edge thereof.
 13. The impeller of claim 9, wherein the chordlength of the splitter blade is from 50% to 80% of the chord length ofthe full blades.
 14. The impeller of claim 9, wherein, at the leadingedge of the splitter blade, the ratio varies from 0.7 at the hub to 0.3at the tip of the splitter blade.
 15. The impeller of claim 10, whereinthe ratio at the leading edge of the splitter blade and at the tip ofthe splitter blade is less than the ratio at the trailing edge of thesplitter blade and at the tip of the splitter blade.
 16. The impeller ofclaim 10, wherein the ratio at the leading edge of the splitter bladevary at a rate that increases toward the tip of the splitter blade. 17.A method for operating an impeller, the method comprising: dividing aflow of working fluid received by the impeller within flow channelscircumferentially distributed around the impeller; for each of the flowchannels: dividing the flow of working fluid in the flow channels into afirst flow channel portion and in a second flow channel portion; and atan upstream location of the first and second flow channel portions,diverging a greater portion of the flow in the second flow channelportion relative to the first flow channel portion by exposing the flowto a greater flowing area in a radially-inward region of the second flowchannel portion than in a radially-inward region of the first flowchannel portion.
 18. The method of claim 17, further comprising, at theupstream location, exposing the flow to a greater flowing area in aradially-outward region of the first flow channel portion than in aradially-outward region of the second flow channel portion.
 19. Themethod of claim 17, further comprising, at a downstream location,exposing the flow to a greater flowing area in a radially-outward regionof the first flow channel portion than in a radially-outward region ofthe second flow channel portion.
 20. The method of claim 17, furthercomprising, at a downstream location, exposing the flow to a flowingarea that is greater in the first flow channel portion than in thesecond flow channel portion.